Ink jet technology continues to be improved in order to increase printing speed and print quality or resolution. One means for improving print speed and quality is to increase the number of nozzle holes in an ink jet printhead and to decrease the diameter of the nozzle holes. However, improvements in print speed and quality often result in operational problems not experienced with lower quality slower speed printers. Such higher resolution printers are more prone to blockages caused by air or debris trapped in critical ink flow areas of the printheads.
In an ink jet printer, ink is provided to the printhead from an ink cartridge or supply tank. The ink flows from the tank through a connecting conduit from the ink cartridge through an ink via in a semiconductor chip or around the edges of a semiconductor chip and into ink flow channels and an ink chamber. The ink chamber is situated in axial alignment with a corresponding nozzle hole and a heater resistor defined on the surface of the semiconductor chip. As electrical impulse energy is applied to the heater resistor, the ink adjacent the resistor is super heated and a vapor bubble forms which propels ink from the nozzle hole onto a print medium. By selective activation of a plurality of heater resistors on a printhead, a pattern of ink dots are applied to the print medium to form an image.
A critical aspect of the printing process is the controlled supply of ink to the printhead. Thermal ink jet printers use a plurality of resistance heating elements in the ink chambers to vaporize a component of the ink which then expands as a vapor bubble forcing ink out of the nozzle associated with the chamber. The pressure also forces ink out of the supply channel and may affect the ink in the ink supply region or ink via feeding supply channels of adjacent ink chambers.
Shortly after firing the heater element, the ink/vapor interface cools and the vapor bubble begins to contract and finally collapses onto the heater surface. As the bubble collapses, the chamber refills with ink from the ink via and ink supply region by capillary action. As the chamber refills, the ink forms a meniscus which undergoes an oscillatory motion. The oscillatory motion of the meniscus tends to pull a small amount of air into the ink chamber and under certain conditions. Additionally, the temperature of the ink increases as it flows from the reservoir into the heater chip then into the ink chambers. Since the solubility of air in the ink decreases with temperature, air in the ink tends to evolve into small bubbles. In either case, the air may be trapped in the chamber or accumulate in the channel or shelf area between the channel and ink via. Once this happens, the performance of the nozzle degrades severely. Trapped air also acts as a shock absorber which reduces the pumping action of the vapor bubble. Accumulation of air on the shelf adjacent the ink supply channels may affect more than one ink supply channel thereby affecting the ability to sufficiently refill the chambers with ink.
Until now, the primary concern with regard to ink flow distribution was debris which could block the ink supply channels. Accordingly, closely spaced structures for filtering ink feeding each ink channel were thought to be required. Such structures are described for example in U.S. Pat. No. 5,463,413 to Ho et al., U.S. Pat. No. 5,734,399 to Weber et al. and U.S. Pat. No. 5,847,737 to Kaufman et al. Conventional filter structures as described in the foregoing patents, while effective for debris do not solve all of the ink flow distribution problems and may have exacerbated other problems, such as air-related problems.
There is a need therefore for improved nozzle plate structures which provide effective ink flow distribution without degrading ink flow due to debris or air entrapment.